On that note I found this article where in my opinion the OPC (aka GPF or PPF) is very well explained and in details. As an example he uses his EU-spec M2C.
https://drivetribe.com/p/the-opf-pan...TEuQR86iJOdecQ
While some lucky owners of certain car brands will still not be affected by all those restrictions and regulations, the last paragraph sums it up well:
"For those of you, who are planning the removal of the OPFs of your beloved new automobile. Be aware that it will make your vehicle non-homologated to the new WLTP and Euro 6 regulations. If you still want to opt for a better exhaust note, then the only way to do this is by working your way from the rear to the front. A slip-on system which only replaces the rear muffler or backbox is the sweet spot for us in the EU! It can often stay within limits and offers a more sporty sound track across the rev range. For anyone else in the rest of the world, you are lucky because you have way more room to play still. For you nothing has changed yet, but that is only a matter of time."